1. Field of the Invention
This invention is concerned with the generation of true boiling point curves for petroleum crude oils. In particular, it is concerned with a novel vacuum distillation apparatus and method of use thereof which eliminates in reproducible manner the discontinuity usually observed with the raw distillation data at the point at which high vacuum distillation begins.
2. Description of the Prior Art
The first major process in refining crude petroleum is fractional distillation to separate the crude into various boiling range cuts. Crudes differ widely in the amount of material that may be recovered having any specified boiling range. Thus, some crudes may contain as much as 75 vol.% straight-run gasoline, while others contain 10 vol.% or less.
It is of considerable importance to design engineers concerned with the design of distillation equipment to know beforehand the boiling point-yield character of the crude oils expected to be processed at a particular refinery. And, needless to say, boiling point-yield characteristics are critically considered in the purchase and trading of crudes.
Petroleum engineers have developed an assay technique for characterizing a crude that permits them to predict how that crude will behave in a particular refinery distillation. This technique requires that a sample of the crude be subject to analytical distillation under prescribed conditions in the laboratory. The first stage of the distillation is made in an atmospheric pressure still, and the temperature required to accumulate incremental fractions, usually about 2% to 4% of the total crude charge, is noted. A nominal vacuum of about 10 mm is commonly applied after the gasoline cut point of about 425.degree. F. is reached to avoid excessive pot temperatures. This stage of the distillation is terminated at about the temperature at which the crude begins to crack, i.e. undergo thermal changes, which is a temperature in the range of about 625.degree. to 725.degree. F., and usually about 650.degree. F. The termination temperature is determined by the operator, and based on prior knowledge of the nature of the crude type that is being evaluated. After terminating the atmospheric distillation, the reduced crude, or atmospheric residuum, as it is commonly called, is transferred to a vacuum distillation apparatus and the distillation continued under high vacuum, for example, at a pressure of about 200 microns. Each temperature required for the accumulation under vacuum of an increment of distillate is transformed by computation to the temperature that would have been observed at atmospheric pressure were not the crude subject to thermal degradation. This computation of course involves an assumption about the temperature-vapor pressure relationship of the incremental fractions. The vacuum distillation is terminated at about 1050.degree. F., at which point about all of the practically recoverable distillate oil has been taken overhead.
The procedure hereinabove described permits the operator to construct a boiling-point yield curve over the entire distillation range and in terms of temperatures all referred to atmospheric pressure. Such curves are known as "true boiling point distillation curves", or simply "TBP curves", and the distillation procedure as a "TBP distillation". A description of useful apparatus is given in A.S.T.M. Method D2892, titled "Distillation of Crude Petroleum", published by the American Society for Testing Materials, Philadephia, Pa., and is incorporated herein by reference.
The manner in which TBP distillations are presently conducted usually results in a large discontinuity of the raw data, computed at 760 mm pressure, at the point at which vacuum distillation begins. Such a discontinuity is illustrated by FIG. 1 of the drawing. The portion of the curve up to 650.degree. F. is exemplary of a plot for the raw 760 mm data from the atmospheric still. The remainder is exemplary for the vacuum distillation. The first increment of vacuum distillate, the increment noted in FIG. 1 as between A and B, is recovered at a temperature much higher than would be expected from an extrapolation of the atmospheric distillation curve. This aberration, herein called a discontinuity, often persists in progressively lesser degree for later increments of distillate until a temperature equivalent to about 850.degree. F. at 760.degree. mm is reached.
There are two known methods for obtaining TBP curves which reduce or eliminate the discontinuity on changing from nominally atmospheric to high vacuum condition. One of these is to blend back a number of the last increments of atmospheric distillate with the atmospheric residuum prior to initiating vacuum distillation. This method is cumbersome and costly. The other method, more commonly practiced, is for the operator to "adjust" the data points and thus force a continuity on the curve. The latter method leaves much to be desired since the "adjustments" are at least to some extent arbitrary, and in some cases require a repetition of the distillation before adjustments can be made with confidence. Thus, the TBP curve prepared by one operator often will not be exactly like the curve prepared by another operator using identical raw data. Unfortunately, these arbitrary adjustments are made in a region of the distillation where important refinery cut points are set.
At this point we wish to note that TBP curves are not intended to precisely simulate refinery operation. Rather, they provide the engineer with a tool which he uses, together with his prior experience, to predict or estimate how a particular crude will behave in an existing refinery or in a distillation unit of particular design. That this tool is imprecise to the extent that it incorporates an element of uncertainty is evident from the foregoing description. It is an object of this invention to provide an improved method for determining the TBP curve of a crude oil. It is a further object of this invention to provide an improved apparatus for vacuum assay of reduced crudes. These and other objects will be evident to those skilled in the art on reading this entire specification including the claims thereof.